The proper evaluation of cytological specimens, such as Pap smears, is of high importance to public health. Cancer of the uterine cervix is the sixth most common malignancy among women. The Pap smear test, introduced by Papanicolaou and Traut in 1943, provides a painless and inexpensive cytological screening method for the early detection of cervical cancer. While this screening test has decreased the mortality associated with cervical cancer by preventing the development of invasive disease, it is nonetheless dependent on the ability of a technician viewing a specimen under a microscope to detect abnormal cells and structures in thick clusters of cells. Success depends in part on the images presented for evaluation. When the image is viewed through a microscope, the thickness and complexity of the specimen, the nature and concentration of the stain, the microscope objective and the light source all influence what is seen.
The conventional evaluation of PAP smears using standard compound microscopes has a number of shortcomings. PAP smears often contain clusters of cells, but the standard compound microscope using axial illumination is particularly ill-suited for clusters of cells. Thick or overlapping images can be distorted because of diffraction of the light and because of absorption by thick sections. Moreover depth of focus is greatly reduced at high power, and it is difficult to visualize structures above or beyond a thin focal plane. Evaluation is made difficult by deficiencies in resolution, contrast, light penetration and sharpness of image.
As a consequence of the difficult and subjective nature of the test, there is a continuing problem with false negative readings and ambiguous readings called "atypical squamous cells of undetermined significance" (ASCUS). False negative readings can occur 5% to 40% of the time, and are 24 times more likely to occur in samples containing fewer than 50 abnormal cells. Pap smear specimens diagnosed as containing ASCUS occur in 1.6%-9% of all specimens. They are significant because 31% of all ASCUS Pap smears will convert to positive readings within six months. Thick cell clusters, scant cellular number and excessive demands on cytopathologists operating with difficult equipment may account for these shortcomings.
Efforts have been made to develop new methods to replace or enhance the efficiency of Pap smear analysis. These efforts include human papilloma virus screening, automated or semi-automated screening by machine (PAPNET), thin preps and the Bethesda system for classifying smears. Unfortunately, to date none of these methods have been shown to adequately enhance the evaluation of Pap smear specimens.